I'm sure they have Saturday Night Fever more than once, and there should be shows every day. It could be an issue with the online reservation system. You can reserve shows on the ship, and they always hold back (I think) 25% of the seats which can only be reserved on the ship, so that might be another reason you can't do online bookings...the seats that can be booked online are already filled.

You need reservations for Chops and Portofino. Again, you can do that on the ship or online. You will have to decide quickly on a time because Chops reservations do fill up quickly, especially after the first day aboard when people who didn't do an online booking will make their dinner reservations with the Chops maitre'd.